Most conventional computer systems maintain an internal clock to keep track of the time of day. Accurate time of day measurements are required in a wide variety of applications such as managing and tracking electronic mail, timing back-ups of data on a network, synchronizing communications between clients and servers, managing multimedia teleconferences, and assigning a correct time to documents or transactions. Since the internal clock is not perfectly accurate, as time goes by the internal time maintained by the internal clock diverges from the external time with which the internal clock was previously synchronized. This time divergence is sometimes called “clock drift”. Typically the internal clock's time drifts away from the time as a linear function of elapsed time since synchronization. To prevent the clock drift from getting too large, from time to time the internal clock is resynchronized with the external reference time. Time services exist which provide accurate time information using an “atomic clock”. One well-known time service is WWV, which broadcasts a Universal Time Signal. Various solutions have been developed to synchronize the time clock of a computer system. A simple method is for a user of the computer system to manually adjust the clock whenever the clock appears to have drifted. This technique, however, is both inconvenient for the user and subject to its own inaccuracies.
Also, time sources such as WWV occasionally introduce “leap seconds” to synchronize their time with the motions of the planet Earth. To prevent error from accumulating due to drift and leap seconds, it is particularly desirable to synchronize the internal clock of a computer system with the external reference time automatically without intervention of a user or network administrator.
Some prior art methods for synchronizing the internal clock employ a round trip scheme in which a computer system sends a synchronization request to a time service and the time service responds by sending a synchronization message. The inaccuracy of time provided to a computer system is directly related to the total elapsed time for the round trip sequence of messages. Thus, the precision with which clock synchronization can be achieved is limited by the time required for the round trip sequence.
A particular type of round trip synchronization, called Probabilistic Clock Synchronization has been used for synchronizing internal times of computer systems with a reference time from an external source via a communications network. The technique is described in Cristian, “Probabilistic Clock Synchronization”, IBM Technical Disclosure Bulletin, Vol. 31, No. 2 (July 1988), p. 91 which is incorporated herein by reference. The basic round trip sequence works as follows: A client station sends a synchronization request at a time t, according to its clock. A time server responds with a message giving a time T, according to the server's time clock. The client station receives the response at a time t′. It is thus established that the server's time T falls somewhere within the time interval between the client stations times t and t′. Preferably, T is synchronized with the midpoint of the interval between t and t′. Thus, the precision of the client station's synchronization is accurate to within (t′−t)/2. If the achieved precision is not considered good enough, the round trip message exchange sequence is repeated. This method provides means for taking the roundtrip delay time into account and, therefore, reduce the error occurring due to the transmission. However, this method requires a substantial amount of processing in real time.
An improved version of “Cristian's” method is disclosed by Cheung et al. in U.S. Pat. No. 5,535,217 issued Jul. 9, 1996, which is incorporated herein by reference. Enhanced precision is achieved by computing a new precision range for the synchronized time based on an intersection between precision intervals of the client station's time and the time server's time. Unfortunately, also this method requires a substantial amount of processing in real time. Furthermore, both methods determine the time elapsed to transmit a message from the time server to the client station as half the round trip delay.
In U.S. Pat. No. 6,023,769 issued Feb. 8, 2000, Gonzalez discloses a method and apparatus for synchronizing an imprecise time clock maintained by a computer system wherein a first time reading is obtained from a local timing service at a precision greater than that of the clock, and a second time reading is obtained from a remote time server at a precision greater than that of the clock. The second time reading from the time server is corrected for delays associated with the transmission over the network using the first time reading from the local timing service. This method may be able to give a more correct estimate of the time elapsed to transmit a message from the remote time server to the client station. However, it requires messages being transmitted between the client station and the local timing service at a known transmission time or at a time less than the precision error of the local timing service.
Furthermore, none of these methods provide means for preventing tampering with the messages, for example, deliberately delaying the messages by a third unauthorized party.
It is an object of the invention to provide a method for synchronizing a timing device of a client station that substantially reduces the error due to round trip delay.
It is further an object of the invention to provide a method for synchronizing a timing device of a client station that substantially reduces real time processing at the time server as well as at the client station.
It is yet another object of the invention to provide a method for synchronizing a timing device of a client station that substantially reduces the risk of tampering by an unauthorized third party.